MONDAY, Sept. 22, 2014 (HealthDay News) -- Pregnant women appear to have an unusually strong immune response to the flu, according to a new study.

And this strong immune response may help explain why pregnant women get sicker from the flu than other healthy adults. The reason: many symptoms of flu are the result of the immune system responding to the virus, the researchers said.

This finding was unexpected because it's generally believed that pregnancy weakens the immune system to keep it from attacking the growing fetus, according to a joint news release from the Stanford University School of Medicine and Lucile Packard Children's Hospital, Stanford.

"We were surprised by the overall finding," said the study's senior author, Dr. Catherine Blish, assistant professor of infectious diseases at Stanford University School of Medicine in the news release. "We now understand that severe influenza in pregnancy is a hyperinflammatory disease rather than a state of immunodeficiency."

In conducting the study, the researchers examined the reactions of immune cells taken from 21 pregnant women and 29 healthy women who weren't pregnant. The cells were taken from samples of the women's blood before and seven days after they received a flu shot. The researchers also tested cells taken from pregnant women six weeks after their baby was born.

In a lab, the women's cells were exposed to two flu viruses: the H1N1 strain that caused the 2009 pandemic and a strain of the seasonal flu, known as H3N2.

The researchers found pregnancy heightened the immune response of two different types of white blood cells (natural killer and T cells) to both strains of the flu. These cells produced more cytokines and chemokines, which are molecules that help attract other immune cells to an infection site, the study authors explained.

"If the chemokine levels are too high, that can bring in too many immune cells," Blish explained. "That's a bad thing in a lung where you need air space." Getting the flu while pregnant increases women's risk for pneumonia and death, she said.

Although pregnant women with the flu are usually treated with drugs to slow the replication of the virus inside their bodies, the study's authors suggested their findings could lead to improved treatments for these women.

"If our finding ends up bearing out in future studies, it opens the possibility that we can develop new immune-modulating treatment approaches in the setting of severe influenza, especially in pregnant women," concluded the study's lead author, Dr. Alexander Kay, instructor in pediatric infectious diseases at Stanford University School of Medicine, in the news release.

Although the study revealed that pregnant women's immune cells have a greater response to the flu, it remains unclear if these immune cells would have a similar response to other viruses.

"I suspect this is peculiar to influenza for a variety of reasons. I wonder if this is an inflammatory pathway that is normally activated later in pregnancy to prepare the body for birth, but that flu happens to overlap with the pathway and aberrantly activates it too early," Blish suggested.

The study's authors said they hoped their findings would remind women about the importance of getting an annual flu shot. "Flu vaccination is very important to avoid this inflammatory response we're seeing," Kay pointed out. "But only 50 percent of pregnant women are currently vaccinated for influenza."

The study was published Sept. 22 in the Proceedings of the National Academy of Sciences.